Ferrite ceramic compositions and method of preparation



United States Patent Oifice 3,150,095 Patented Sept. 22, 1964 3,150,095FERRITE CERAMIC COMPOSITIONS AND METHOD OF PREPARATION James M.Brownlow, Crompond, and James A. Morrow, Peekskill, N.Y., assignors toInternational Business Machines Corporation, New York, N.Y., acorporation of New York No Drawing. Filed Nov. 24, 1961, Ser. No.154,896

8 Claims. (Cl. 252-625) This invention relates to the ferritecompositions which are used in memory and switching circuits in computermechanisms. These ferrite compositions find application as high speedswitching elements in computer circuitry.

In formulating magnetic materials for computer use there are severalwell recognized objectives in regard to the properties which arerequired to permit operation of the material as a high speed memorydevice. These properties are not independent of the several electricalmodes of operation that computer designers have developed. In oldercomputer memory designs it was only necessary that the magnetic ferritehave a high B /B ratio and a corresponding high one to zero (signal tonoise ratio) in the memory circuit. The switching speed and flux densitywere not important limitations since the repetition rates were low, 100kilocycles.

In present day memory design the switching speeds and repetition ratesare being pushed to the highest limits by more sophisticated circuitdesign and modification in materials.

It has been found the ordinary square loop ferrite compositions withfiux densities 1800-2700 ganss, exhibit a thermal effect at fast (25megacycles) repetition rates, Hysteresis heating on repetitive switchingat high rates causes the temperature to rise excessively in the ferritecomposition. The power (P) dissipated in the material at a fixedrepetition rate is proportional to the flux density (B times thecoercive force (H [i.e. P-(B H The coercive force (H can not be loweredtoo far without loss of switching speed. If the flux density is lowered,stable memory operation is possible because the temperature rise due tohysteresis heating is then within safe limits.

An object of this invention is to provide ferrite compositions andmethod of making them so that low flux density is achieved whileretaining the desirable properties of fast switching and high B /Bratio.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodi ment of thp invention.

It is also desirable to keep the Curie temperature high so that thematerial can sustain stable operation when the temperature does rise.This eliminates one way of lowering the fiux i.e. by the substitution ofzinc or cadmium in Fe O which lowers the Curie temperature and therebythe flux density if the Curie temperature is near 100 C. (SaturationMagnetization and Crystal Chemistry of Ferrimagnetic Oxides by E. W.Gorter, Philips Research Reports, vol. 9, pp. 295420). Such materialsare unsuitable because a large temperature rise causes them to approachthe Curie temperature where memory operation becomes impossible.

The approach used to solve this problem has been to adjust thecomposition of the ferrite so that the flux is lowered while the Curietemperature remains high. This requires additions to Fe O which dilutethe magnetic moment but do not greatly diminish the number of mag neticinter-actions. It was found the Cr+ and Cu+ act in this way and that Ni+in small amounts promotes high B /B ratio.

These materials disclosed have been made and tested under memoryconditions and found to dissipate less power at high repetition rates(2-5 megacycles) than conventional compositions. They have low magneticmoments, switch at high speed and are useable at 5 megacycles whereconventional ferrite compositions are not suitable.

The formulation and processing of these materials follows procedureswell known in the art except for the firing step. Here a problem arisesbecause of the high copper content. At 1150l200 C. the Cu+ disassociatespartly into Cu Too great an amount of disassociation leads to chemicalinstability and a foreign phase is formed which results in a 2 phasematerial with reduced B /B ratio. Careful experimentation with eachcomposition allows a narrow range of firing temperature which is withini50 C. of the optimum firing temperature to be defined for each. In somecases a two step cooling enhances the B /B ratio.

While the firing is critical, the invention is in the unique compositionof the ferrite which has a desired combination of magnetic propertiesnamely low fiux density (300- 1600 gauss), high B /B ratio (.7.9), lowhysteresis heating effect of the order of /3 to /2 that found in highflux density ferrite composition and capable of operating at highrepetition rates (2-5 megacycles) and a Curie temperature (T greaterthan 150 C.

The range of ferrite ceramic compositions found to be within the scopeof the invention are described by the formula expressed in atom numbers:

Fe Cr Cu Mg Ni m wherein y .2-1 z: .5-.8 a=0.2 Zr==0.4

and

A ferrite ceramic composition is defined and understood by those skilledin the art to be a material with a cubic spinel structure which resultsfrom the heating (firing) and complete reaction of the component oxidesused in preparing the ferrite composition.

The ferrite ceramic composition of the invention may be prepared bymixing together oxides of Fe, Cr, Cu, Mg, and Ni in the amounts shownbelow in Table II to form a mixture and then subjecting this mixture toan elevated firing temperature from 1100 to 1400 C. for up to 60 hoursin an atmosphere containing oxygen to form the ferrite ceramiccomposition. Thereafter the ferrite ceramic composition may be cooled byair quenching. In some cases a second firing step is used involving arapid reheating to a temperature lower than the first firing temperaturefor a specific time and then air quenching. Ordinarily in the two stepmethod the second firing temperature is to 400 lower than the firstfiring temperature and the second firing time varying from 10 minutes to2 hours.

The initial mixture is prepared by weighing out the component oxides asspecified for any formula number specified in Table II. This mixture ishomogenized for four hours in a ball mill with alcohol (e.g. ethylalcohol) as the suspending agent. The alcohol is removed by drying andthe mixture is calcined at 1000 C. for a time of 1 hour. This calcinedmixture is again milled in a ball mill with water and 3% by weight of abinder (e.g. polyvinyl alcohol) for a period of time (between four and24 hours) suificient to reduce the particle size of the calcined mixtureto about 1 micron. This material is then dried and reduced to a powderand the powder used to form toroidal sample shapes by pressing in asteel die of suitable design at a pressure of the order of 50,000 lbs.per square inch. Next the toroidal shapes or bodies are fired and cooledaccording to the temperature and time schedule specified for use inpreparing the initial mixture as set forth in Table I. Thus ferriteceramic compositions are formed which have a cubic spinel' structure.

Table I INITIAL MIXTURE IN GRAMS AND FIRING TREATMENT Weight in gramsStep 1* Step 11* Formula No. Temp, Time, Temp, Time,

F6203 CrzO: CuO MgO NiO 0. hrs. 0. hrs.

T 390. 144.0 38.0 55.8 0. 1,200 0.5 T 91.... 120.0 53.2 63.6 0.0 1,2000.5 T 393- 95. 0 113.8 39. 7 0. 0 1,200 0.5 T 394. 120.0 0.0 39.7 0.01,200 16.0 T 95 96.0 68.4 55.8 14.9 1,200 16.0 T 96 80.0 83.6 55.8 14. 91,200 16.0 T 9 9 120.0 ,60. 0 39.7 14.9 1,200 16.0 T l00 88.0 83. 6 47.7' 1419 1,200 16.0 T 103--. 144. 0 38. 0 39. 7 14.9 1,100 00. 0 T 107-"144. 0 30.4 31. 8 30.0 1,400 0.15 '1 108-.. 144.0 38.0 23.9 22.5' 1,2003.0 T 1 12 144.0 19.0 47. 7' 11.2 1,150 4.0 T 113 144. 0 15.0 59. 7 7 1,200 16. 0 T 114-" 120.0 38.0 59. 7 7.5 l, 150 16.0

Firing treatment: Firing done in air atmosphere. air quenched alter eachstep.

The samples are Table II FINAL COMPOSITIONS IN ATOM NUMBERS ANDLEAGNETIG PROPERTIES FOR SYSTEM FeXCr QuzMgaNib Atom numbers FluxFormu1a* density B lB He, '1

o. s oersted C.

x y z a b gauss *These are the final ferrite ceramic compositionsobtained by treating mixtures given in Table 1 according to firingtreatment set forth therein.

"The unique composition ofzthe ferrite ceramic compositions causes themto exhibit a highly desirable comhination of magnetic properties, namelylow flux density (3001600 gauss), high B /B ratio (.7.9), low hysteresisheatingetfect of the order of /3' to that found in high flux densityferrite ceramic compositions that are the invention acomparison of oneof the specific compositions of the invention witha manganese containingferrite i composition currently available in the art is provided a Table111.

4 Table 111 Type XCll.1 Mure F614 04 T 99C11.5 C Fens N12 04 Wherein 11is the percent flux switched in a partial switching memory operation. 7V v v I Power expressed in watts is a measure ofheat dissipated in themagnetic ferrite composition at the high repetition rate expressed inmegacycles (me).

The invention shows that low flux density ferrites dissipate less heatat high repetition rates. The Table III shows that the power dissipatedat 2 me. is .027 watt in T 99 compared to .073 watt in the high fluxdensity reference material. A similar relation is seen at 5 me. It hasbeen found that the reference material (TypeX) will not operate at 5 mo.unless a liquid refrigerant is forced over the material. The T 99 andother ferrite compositions of this invention will, however, operate inair at this repetition rate. I

In large capacity high speed memory is highly desirable that it operatein air and-without extensive refrigeration.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1-.- A- ferrite ceramic composition having the formula Fe cr cu Mg Ni ogwherein x=0.8-1-.8 y=0.21.3 z=0.5-'0;8 a=00-.2" b=0-0.4

and

x+y+z+a+l1=3 2113+ y-2'.3 0.7 a+b,+z1.0

said composition having a flux density (B of less than 1600 gauss, a B/B ratio of at least 0.7 and a T of at least C.

2-. A ferrite ceramic composition having the formula Fe Cr Cu Mg Ni Owherein x=1.21.8 y='0.5-1.3 z='0;5-0.8 a=0 [2:0

and

said composition having a flux density (B of less than 1600 gauss, a B'/Bg ratio of atleast 07 and: a T of at least 150 C. r l i 3. A ferriteceramic composition having the formula and havinga'flux density (B of770 gauss, a B /li i ratio of 0.8 anda-T, of C. v 4. A ferrite ceramic,composition havingthe formula v r rt a ttM tz r and having a fluxdensity (B of 740 gauss', a B /B ratio of 0.8 and a T of 175 C.

5. A ferrite ceramic composition having the formula and having a fluxdesity (B of 1100 gauss, a B /B ratio of 0.9 and a T of 200 C.

6. A ferrite ceramic composition having the formula r.s o.25 0.6 o.2o.15 4

and having a flux density (B of 1260 gauss, a B /B ratio of 0.9 and a Tof 280 C.

7. A method of preparing a ferrite ceramic composition having a fluxdensity (13,) of less than 1600 gauss,

a B /B ratio of at least 0.7 and a T of at least 150 C.

having the formula which comprises mixing in finely divided form oxidesof Fe, Cr, Cu, Mg and Ni in proportions such that the ferrite ceramiccomposition produced by firing has the above formula; subjecting thethus-formed mixture to an elevated firing temperature oetween 11001400C. for up to hours in an oxygen containing atmosphere to form saidferrite ceramic composition and thereafter air quenching.

8. The method of claim 7 wherein there is a second firing step followingthe air quenching which comprises a rapid reheating in an oxygenatmosphere to a temperature to 400 C. lower than the first firingtemperature, holding at this lower temperature for 9 minutes to 2 hours,and thereafter air quenching and References Cited in the file of thispatent UNITED STATES PATENTS 3,031,405 Pierrot et al Apr. 24, 1962FOREIGN PATENTS 760,035 Great Britain Oct. 31, 1956

1. A FERRITE CERAMIC COMPOSITION HAVING THE FORMULA